1. Field of the Invention
The present invention relates to a costas loop and a data identification apparatus. More specifically, the present invention relates to a costas loop which demodulates a BPSK-modulated signal, and a data identification apparatus which identifies whether or not predetermined data is included in a demodulated signal.
2. Description of the Prior Art
One example of a kind of a complex PLL called a costas loop after Costas, J. P., or an I-Q loop is disclosed in pages 166-168 of a book "Spread Spectrum System" (R. C. Dixon, translated by Tatsuno Satoshi et al.) JATEC. With referring to FIG. 16, in a costas loop 1, a BPSK-modulated signal is multiplied by an oscillation signal from a VCO (Voltage Controlled Oscillator) 3 by a multiplier 2a, and multiplied by the oscillation signal being phase-shifted by a 90.degree. phase-shifter 4 by a multiplier 2b. Outputs from the multipliers 2a and 2b are filtered by low-pass filters (LPF) 5a and 5b, respectively, and an output from the LPF 5a is output as a BPSK-demodulated signal, and the outputs from the LPFs 5a and 5b are multiplied with each other by a multiplier 2c. Then, an output of the multiplier 2c is applied to the VCO 3 via a loop filter 6, whereby an oscillation frequency of the VCO 3 is controlled.
The BPSK-modulated signal is represented as ".+-.A cos (.omega. t)", the output of the VCO 3 is represented as "cos (.omega. t+.increment.)". In addition, ".increment." is an error of phase-shift. Therefore, the output of the multiplier 2a becomes ".+-.A(cos.increment.+cos (2.omega. t+.increment.))/2", and when the output is filtered by the LPF 5a, a high-frequency component included therein is removed, ".+-.Acos.increment./2" remains. This component can be approximated to "A/2" when ".increment." becomes zero, being demodulated data.
On the other hand, the output of the LPF 5b becomes ".+-.sin .increment./2" as similar to the above. If the output of the LPF 5a is multiplied by the output of the LPF 5b by the multiplier 2c, a multiplication result becomes "sin (2.increment.)/4". Therefore, when ".increment." is small, "sin (2.increment.)/4.apprxeq..increment./2" is obtained, and therefore, it is possible to control the VCO 3 such that the ".increment." becomes zero. In addition, a constant for a control system is set in the loop filter 6.
Such a costas loop 1 is almost constructed by analog circuits, and therefore, it is difficult to incorporate the costas loop 1 in an integrated circuit. Furthermore, since the costas loop 1 is constructed by analog circuits, there were problems that troublesome adjustment is required and that an influence of a change of temperature is large.